I know this is about MongoDB in action not MongoDB in reference. But something I felt missing when I was trying to actually write some of my own code was a nice list of the main commands to be used. I had to go online and find other cheat sheets and reference lists so I could quickly review the different queries and commands. e.g find(), findOne(), insert(), $update, $remove, $gt $gte, $lt, $exists, $regex, use <db>, show collections, etc..

Whilst you explain all the commands and queries and give examples, trying to remember which one, or quickly being able to review which I can use so I can construct something myself is hard.

I'm not sure if you've still got time to add a reference list of commands into the 2nd edition of the book, but it'd be great if you could! 2 or 3 pages worth would cover pretty much everything!